1. Field of the Invention
The invention concerns the transmission of signals between an earth-station and a spacecraft in geostationary orbit.
2. Description of the Prior Art
There are already many stabilized satellites in an orbit such that the satellites remain at all times vertically above a particular place on the Earth. Such satellites are called geostationary satellites for short.
Geostationary satellites are usually remote sensing satellites or telecommunication satellites.
In fulfilling their mission, geostationary satellites exchange signals with the Earth.
Geostationary satellites are sometimes replaced with groups of small satellites with one serving as a relay station for the others, for example as described in French Patent 2,539,102.
Because of the large number of geostationary satellites and because, to remain geostationary, they must all be in the same orbit (an equatorial orbit at 36,000 km altitude), very strict international rules have been drawn up to cover both their positioning and the methods of earth-satellite transmission. In particular, these rules allocate different frequencies for uplink transmission (from the Earth to the satellite) and for downlink transmission (from the satellite to the Earth); the main reason
These international rules are embodied in an international treaty which is legally binding on all member states of the International Telecommunications Union, which includes France.
The first frequency bands allocated to satellite operators were chosen in the "low" frequencies, i.e. in the C band (typically from 4 GHz to 6 GHz). Once this band had become saturated, the frequency bands allocated were chosen in progressively higher bands (including the Ku band (beyond 12 GHz) and even the Ka band (from 20 GHz to 30 GHz)).
One constraint arising out of the increasing values of the frequencies allocated is that increasing the frequency requires an increase in transmission power (among other reasons because of absorption by the atmosphere) and therefore more sophisticated and more costly technology.
Each satellite is allocated a "slot" in the orbit defined with an angular accuracy (as seen from the Earth) of .+-.0.1.degree. (which is equivalent to a parallelepiped 180 km long with a cross section 80 km.times.80 km) and two frequencies different from those allocated to the adjoining slots and different from each other to make the uplink and downlink independent. These frequencies are allocated in bands 250 MHz wide but because the ranges and frequencies available are finite and the numbers of signals to be transmitted in a band and the multiplexing capacities are limited the narrowness of the bands allocated has not prevented saturation and the need for ever higher frequencies.
An object of the invention is to enable use of the same frequency for the uplink and the downlink, without risk of interferences, to again make available frequency bands in the "low" frequencies, especially but not necessarily in the C band, and to recognize the ever increasing demand for frequencies.